Treatment of sodium base sulfite residual liquor



May 14, 1957 TREATMENT OF SODIUM BASE SULFITE RESIDUAL LIQUOR Filed April 1, 1954 3 Sheets-Sheet 1 g l 7?; Cookinq Waste *3 L|quar Gases d I L golllttlbp f? 0n pin/n fr ris llglgm l g e 252 5 I u I e 24% 572122- C ookin g Process l nfqwgl l bhur Y RECOVER/N6 50 Valence AND REGENERAT- pH ADJUS TING TREATMENT flfggg Treated M/xture JEPARA TIN 6 SUSPENDED EVAPORA TING ggrgl bgstl'on 50L/D 5 C I d 00 e C I Contain/n Mifififiew s 0 q P/Gases BURNING v COOL [N6 5 ell C [0' NZ S mfg NZZ'E'Q TREAT/N 6 SNELT 7'0 REC OVER .Sodium SODIUM COMPOUNDS gg gqggflgg CONTAIN/N6 5mm gang/4m I v Sal/mg AND CARBONIITE gg m a m reatrent q Smelt Water 5ELEC TIVE D15 5 OLV/N 6 Na 00 Salullbn 5015x131 COnl'dl'nlh Na 5 q Undiss olved s INVENTORS ATYoRNEYS.

y 4, 1957 L. BRADLEY ETAL 2,792,350

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ATTORNEYS.

May 14, 1957 L. BRADLEY ET AL 2,792,350

TREATMENT OF SODIUM BASE SULFITE RESIDUAL LIQUOR Filed April 1, 1954 3 Sheets-Sheet 3 51114211 WaZer-- DISSOLVING 4 Lime Green Liquor Clarified i 8 53 7 7 il g Green L/quor PIPEG/P/M T/IVG 6 PARA TIA/6 E Na aim/M60 2/01 hzeriiquor waw Sa/ulion Con t am/nq Na .5 3 PPECI Pl TA TIN 6 SEPARA T/NG Solution a /NaHC0 3 "7 7'eati1 1 g7 Z Smelt Al! or ofher f Oxidiz/nq Agent SELECTIVE Water DIS 50L V/NG /5/urry SEPA RATING WaIer Na C0 Na CO Solution So/ut/on Containinq was ATTORNEYS.

United States Patent G TREATMENT OF SUDIUM BASE. SULFITE RESIDUAL LIQUOR Linn Bradley, Montclair, N. J., and Robert Q. Boyer, Los Angeles, Calif., assignors to Western Precipitation Forporation, Los Angeles, Calif., a corporation of Caliorma Application April 1, 1954, Serial No. 420,307

15 Claims. (Cl. 252-183) This invention relates to the treatment of residual liquor from the digestion or cooking of wood or other lignin bearing cellulosic material with sodium base sulfite cooking liquor comprising an aqueous solution containing a sulfite compound of sodium. It relates more particularly to methods of treating such residual liquor to recover sulfur and sodium contained therein, and to the regeneration of the sulfite compound of sodium in suitable form for re-use in the cooking process.

The residual liquor from the cooking process contains sulfur dioxide bearing compounds of sodium, principally as ligno-sulfonic compounds or other organic sulfonic compounds formed by action of the cooking liquor on organic substances present in the lignin bearing cellulosic material during the cooking process, together with some residual sulfite compound or compounds. It also contains other organic compounds derived from the lignin bearing cellulosic material or formed by action of the cooking liquor on constituents of such material including, for example, sugars or other carbohydrates, and various organic acids, such as formic or acetic acid, or salts thereof.

The process of this invention is applicable to treatment of residual liquor obtained with various types of sodium base sulfite cooking processes, such as:

Sodium base acid sulfite processes, in which the cooking liquor contains sodium acid sulfite (NaHSOs) and generally also contains free sulfurous acid, and the residual liquor contains some sodium acid sulfite together with ligno-sulfonic compounds and other organic compounds such as mentioned above;

Neutral or alkaline sodium base sulfite processes, such as the neutral sulfite semi-chemical process, and the processes known as Keebra or sodite processes, in which the cooking liquor contains normal sodium sulfite (NazSOs) and one or more other alkaline compounds of sodium such as sodium bicarbonate, carbonate or hydroxide, and the residual liquor also contains sodium sulfite and some of the other alkaline sodium compound together with ligno-sulfonic compounds and other organic compounds such as mentioned above.

The residual liquor from such sodium base sulfite cooking processes generally also contains one or more soluble calcium compounds due to the presence of calcium derived from the lignin bearing cellulosic material and from calcium salts present in the water used in the cooking liquor, the amount of such calcium compounds in the residual liquor being relatively small compared to the amount of sodium compounds.

This application is a continuation-in-part of our application Ser. No. 346,572, filed April 13, 1953, now abandoned.

The principal object of the invention is to provide an economical process of treating residual liquor from such cooking processes to recover therefrom a large proportion of the sodium and sulfur contained therein, in forms suitable for re-use in the cooking process, and to also utilize ice A particular object of the invention is to react added,

sulfur bearing material with the residual liquor prior to the evaporation step, in such manner as to lower the sulfite ion concentration, provide a treated mixture substantially free from volatile sulfur compounds, suitably adjust and stablize the pH of the mixture, and produce a favorable ratio of sulfur to sodium in the concentrated mixture obtained by evaporation of water, in order to reduce thev corrosive nature of the mixture, prevent the release of sulfur dioxide or other volatile sulfur compounds during evaporation, provide a treated mixture which can be concentrated to a high degree without precipitation, provide a concentrated mixture which is easily flammable in a burning operation in a modern Kraft recovery unit, andpromote the release of sulfur as sulfur dioxide in the burning operation.

Another particular object is to substantially prevent the release of volatile sulfur compounds from the mixture during the evaporation step.

Another object is to reduce the quantity of corrosive gases given 01? during the evaporation.

A further object of the invention is to provide for the recovery of compounds of sodium and sulfur from the residual liquor and for the utilization of compounds so recovered in the production of a relatively pure solution containing a sulfite compound of sodium suitable for use in cooking liquor.

Another object of the invention is to provide for treat ing the residual liquor with an added fixing agent prior to evaporation, in such manner as to provide a desirable ratio of sodium to sulfur so that, upon concentration and burning, suitably related proportions of sulfur will be released as sulfur dioxide and recovered as sodium sulfide in the smelt, while the smelt will also contain a suitable quantity of sodium in the form of sodium car-. bonate. In this connection, a particular object is to produce, in the burning operation, combustion gases containing sulfur dioxide and a smelt containing sodium sulfide and sodium carbonate, the proportions of such sulfur dioxide, sodium sulfide and sodium carbonate being such that the sodium sulfide may be used as a fixing agent in the treatment of a further quantity of residual liquor, and the sodium carbonate may be utilized in re covering the sulfur dioxide from the combustion gases to regenerate sodium sulfite cooking liquor. In certain preferred embodiments of the invention, the proportion of sodium carbonate contained in the smelt is such that a portion thereof may also be utilized along with the sodium sulfide in the treatment of the residual liquor.

Another object of the invention is to provide for removal of substantially all of the calcium contained in the form of soluble compounds in the residual liquor, so as to prevent mechanical clogging, plugging or scaling of Q-o lowing description with reference to the accompanying drawings, in which:

Fig. l is a fiow sheet illustrating the recovery process in accordance with the invention;

Fig. 2 is a fiow sheet illustrating more particularly the steps that may be used according to certain preferred embodiments of the process; and

Figs. 3, 4 and 5 are partial flow sheets illustrating other modifications of the process.

In order to carry out the process of the invention, the residual liquor from the digestion of lignin bearing cellulosic material with sodium base sulfite cooking liquor is separated from the pulp and from other undissolved solids discharged from the digestor, by any suitable method such as ordinarily employed for this purpose. The pulp is preferably washed with water, and part or all of the washings may be combined with the residual liquor. This residual liquor contains Sulfur dioxide bearing compounds and other constituents, as described above.

Inaccordance with the present invention, as illustrated generally in Fig. 1, this residual liquor from the cooking process, after separation from the pulp, is mixed with an alkaline fixing agent comprising sulfur bearing material, and the resulting mixture is subjected to a fixing and pH adjusting treatment to neutralize any acid constituents thereof, to so alter the chemical composition thereof as to substantially prevent loss of volatile sulfur compounds during subsequent evaporation of the treated mixture and facilitate the recovery of sulfur and sodium therefrom in the form of useful compounds, and to adjust and stabilize the pH of the mixture at a value suitable for evaporation.

This fixing and pH adjusting treatment comprises treating the mixture for a sufficient period of time and under such conditions as to cause reaction between the added sulfur bearing material and sulfur dioxide bearing constituents of the residual liquor and thereby adjust the hydrogen ion concentration of the mixture and remove volatile sulfur compounds present in the residual liquor or contained in the added fixing agent. The principal sulfur compounds present in the residual liquor are sulfur dioxide bearing compounds of sodium such as ligno-sulfonic compounds and sulfite compounds, which contain sulfur in the tetravalent state. These tetravalent sulfur compounds are volatile upon heating, that is, they would volatilize or decompose with evolution of sulfur dioxide if they were allowed to remain in the liquor during heating to evaporate water therefrom. The fixing agent added to the liquor comprises, as an essential ingredient, a material containing sulfur in an oxidation state lower than tetravalent and capable of reacting with the tetravalent sulfur compounds present in the liquor to form non-volatile compounds. For example, the fixing agent may contain sodium sulfide, sodium hydrosulfide, hydrogen sulfide, sodium thiosulfate, colloidal sulfur, or a sodium polysulfide. Sodium sulfide, sodium hydrosulfide, hydrogen sulfide and sodium polysulfide are sulfide compounds from which hydrogen sulfide is releasable by evaporation and concentration of the mixture containing the fixing agent. The fixing agent may contain one or more than one such material containing sulfur in an oxidation state lower than tetravalent. It may also contain one or more alkaline compounds of sodium which do not contain sulfur,;such as sodium hydroxide, sodium carbonate, or sodium bicarbonate. The fixing agent may be added in the form of an aqueous solution, or as an aqueous slurry, or in solid form. I

i In accordance with our invention, the alkaline fixing agent added to the residual liquor contains a sufficient amount of one or more sulfur bearing materials such as mentioned above to provide between /2 and 1 /2 parts by weight, and preferably between about part'and about 1 part by weight, of sulfur in an oxidation state lower than tetravalent for each part by weight of tetravalent sulfur present in the residual liquor.- The composition and amount of this added fixing agent should also be such that the sum of the alkaline sodium in the fixing agent and in the residual liquor equals between about 1 /2 and about 3 parts by weight for each part by weight of tetravalent sulfur in the residual liquor. The term alkaline sodium means the sodium in the form of alkaline compounds, as determined by titration with a strong acid to a methyl orange end point. Such alkaline sodium may include, for example, sodium in the form of sulfide, hydrosulfide, polysulfide, hydroxide, carbonate or bicarbonate in the added fixing agent. In the case of residual liquor from a neutral or alkaline sodium base sulfite cooking, process, such alkaline sodium may also include, for example, sodium in the form of bicarbonate, carbonate, hydroxide, or salts of organic acids such as the acetate, in such residual liquor, as well as one-half of the sodium in the form of neutral sulfite (NazSOs) in such residual liquor.

The mixture of residual, liquor and fixing agent is subjected to a fixing and pH adjusting treatment for a sixthcient period of time and under such conditions as to cause the added material containing sulfur in a state of oxidation lower than tetravalent, and the volatile sulfur dioxide bearing compounds contained in the residual liquor to react substantially completely to form compounds which will not volatilize upon subsequent heating of the mixture to evaporate water therefrom, and thereby produce a treated mixture substantially free from volatile sulfur compounds, that is, sulfur compounds which upon heating to evaporate water from the mixture would volatilize or decompose with evolution of gaseous sulfur bearing products.

. The reactions that take place between thev tetravalent sulfur compounds in the residual liquor and the sulfur bearing material in the fixing agent, during this fixing and pH adjusting treatment, involve reduction of the tetravalent sulfur to a lower state of oxidation, and oxidation of'the sulfur in the added sulfur bearing material to a higher state of oxidation. These oxidation-reduction reactions result in the formation of reaction products that will not volatilize or decompose with evolutionof gaseous sulfur bearing products upon subsequent heating of the mixture to evaporate water therefrom, and they remove from the mixture the volatile tetravalent sulfur compounds of the-residual liquor as well as volatile sulfur' compounds contained in the added fixing agent, such as hydrogen sulfide or other sulfide compounds which would evolve hydrogen sulfide upon heating the mixture to evaporate water therefrom.

For the purposes of our invention, it is important to cause these oxidation-reduction reactions which result in the removal of volatile sulfur compounds to occur substantially completely in the mixture prior to evaporation. The temperature normally prevaling in the residual liquor after separation fromv the pulp is generally in a range somewhat above atmospheric temperature but below F. If the residual liquor at a temperature within this normally prevailing range is simply mixed with a suitable proportion of the above described fixing agent containing sulfur in a state of oxidation lower than tetravalent, these oxidation-reduction reactions do not occur readily, and there is little or no change instate of oxidation of sulfur or removal of volatile sulfur compounds from the mixture under these conditions even though the mixture is allowed to stand for. a very long period of time. Under these conditions the mixture is still corrosive and contains substantial quantities of volatile sulfur bearing compounds. Consequently if the-mixture without further treatment were heated to concentrate it by evaporation of water, a serious loss of sulfur in the form of evolved gaseous products would result, and the corrosive nature of the mixture and the evolved gases would result in serious corrosion of the equipment or necessitate the use of expensive corrosion-resistant equipment.

It is therefore necessary to subject the mixture to a special fixing treatment which will cause the desired oxidation-reductions to occur more rapidly and completely and thereby remove the volatile sulfur compounds from the mixture prior to evaporation. This fixing treatment may comprise, for example, heating the mixture under pressure at a temperature of about 300 F. or higher, or aerating the mixture by contact with air in such manner as to provide an extended surface of contact of the air with the mixture, or may comprise both heating under pressure and aeration, as described more fully hereinafter, and such treatment is continued for a sufiicient period of time to cause the sulfur bearing material in the added fixing agent to react substantially completely with the sulfur dioxide bearing constituents of the residual liquor to remove volatile sulfur bearing compounds as described above.

The above-mentioned oxidation-reduction reactions also serve to adjust the pH of the mixture. When the alkaline fixing agent is mixed with the residual liquor, the pH initially established in the resulting mixture is higher than that of the residual liquor itself, and some acid-base reactions take place between constituents of the residual liquor and the fixing agent. However, these acid-base reactions do not change the oxidation state of the sulfur present and are therefore to be distinguished from the above mentioned oxidation-reduction reactions. The oxidation-reduction reactions caused by treating the mixture as described above result in a further adjustment of the pH by reducing the hydrogen ion concentration. There may be other reactions during this fixing treatment that are alkali consuming, such as reactions between alkaline sodium compounds and organic compounds, or the formation of organic acids during such treatment. These alkali consuming reactions may partly or wholly offset the effect of reduction of hydrogen ion concentration by the oxidation-reactions, or may even result in a lowering of the pH below that initially established in the mixture prior to the fixing treatment. An important advantage of the fixing treatment is that it causes all of these reactions which tend to change the pH of the mixture to be substantially completed prior to the evaporation steps, so as to establish in the treated mixture a desired pH which is suitable for evaporation and will remain stable during the evaporation. In order to permit evaporation without abnormal change in pH, without release of volatile sulfur compounds, and without precipitation of organic compounds, the composition and amount of the added fixing agent and the conditions and duration of the pH adjusting and fixing treatment of the resulting mixture should be such as to establish in the treated mixture a stable pH greater than 9, and preferably greater than 10. The establishment of a stable pH of about 11 is especially advantageous, particularly since it permits the treated mixture to be concentrated by evaporation to about 70% solids or more without appreciable precipitation of organic compounds.

The particular chemical reactions that take place during this fixing and pH adjusting treatment depend upon various factors such as the composition of the residual liquor, the composition and amount of the added fixing agent, and the conditions of treatment. The invention is not limited to any particular chemical reaction or reactions in this stage of the process, but the following are examples of some of the oxidation-reduction reactions that may occur and which result in the removal of volatile sulfur compounds and the stabilization of the pH as described above.

If the added fixing agent contains a sulfide compound, the possible reactions may be considered stepwise as follows:

.If the treatment is carried out in the presence of air, this first step may include other reactions, such as:

'II. In the presence of some alkali, the following reactions may occur:

Ill. Depending upon the pH of the solution, one or more of the following reactions may occur:

IV. Further possible steps in the reactions, resulting in formation of polythionates and reduction of hydrogen ion concentration, are as follows:

If the fixing agent contains colloidal sulfur or a sodium polysulfide, reactions may occur as set forth in steps Ill :and IV above, while with sodium thiosulfate reactions may occur as in step IV above.

There is an equilibrium between ligno-sulfonic groups on the one hand, and the bisulfite ion and the lignin groups on the other hand, for example, as follows:

Therefore, the removal of bisulfite ion from the solution, as by the illustrative reactions outlined above, effectively removes or destroys ligno-sulfonic compounds. Some of the ligno-sulfonic compounds may also be converted to sulfo-lignin or hydroxylignin compounds, as by reaction of the organic group with sodium hydrosulfide and sodium hydroxide, with the release of bisulfite ion for removal from the solution by reactions such as set forth above. It is also possible that reactions similar to some of those outlined above may occur within the ligno-sulfonic compounds themselves, followed by decomposition to form comparable products.

The treated mixture produced by this fixing and pH adjusting treatment is then concentrated by heating to evaporate water therefrom While retaining substantially all the sodium and sulfur present therein. If desired, a sulfide or other sulfur compound of sodium may be introduced into the concentrated mixture. The concentrated mixture is burned in a suitable furnace to produce heat by combustion of organic constituents, to convert part of the sulfur to sulfur dioxide which is discharged in the combustion gases, and to produce a smelt containing the sodium and most of the remaining sulfur, principally in the form of sodium sulfide and sodium carbonate.

In the preferred practice of the invention, the smelt so produced is treated to recover a sodium carbonate compound and a sodium compound containing sulfur in a state of oxidation lower than tetravalent. At least part of the sodium carbonate compound so recovered is preferably utilized in such manner as to provide for recovery of sulfur dioxide from the combustion gases from the taining' a sulfite compound of sodium for re-use in the cooking process, the combustion gasesbeing first cooled, and treated to separate suspended solids, prior to treatment to recover sulfur dioxide therefrom. Part or all of the sulfur bearing compound of sodium recovered from the smelt is preferably utilized in" the above-mentioned fixing and pH adjusting treatment, and part of the sodium carbonate contained in the smelt may also be recovered either with the sulfur bearing sodium compound or separately therefrom and utilized in the fixing and pH adjusting treatment.

If desired, part ofthe" sulfur bearing compound of sodium recovered from the smelt may be used for other purposes, such as the preparation of S02, NaaSOz; or NazCOa, or as a neutralizing agentor otherwise in processes known to those skilled in the art,.or may be treated so that it may be recycled to the furnace with release of a considerable proportion of its contained sulfur.

Certain preferred embodiments of the process are illustrated more fully in Fig. 2, as applied to the treatment of sodium base sulfite residual liquor such as described above, from either an acid, neutral or alkaline cooking process, and containing a small proportion of soluble calcium compounds. After separation from the pulp, the residual liquor is mixed with an alkaline fixing agent containing an alkaline sulfide compound of sodium, such as sodium sulfide (NazS) or sodium hydrosulfide (NaHS) or both, and also containing sodium carbonate. The alkaline fixing agent may also contain some other alkaline sodium compound such as sodium bicarbonate or hydroxide, but the sodium is preferably present therein principally or substantially Wholly as a sulfide compound and sodium carbonate. This alkaline fixing agent is preferably added to the liquor in the form of an aqueous solution, but may if desired be introduced partly or wholly in solid form or as an aqueous slurry.

After introduction of the alkaline fixing agent in an amount at least sufficient to cause precipitation, in the form of calcium carbonate, of substantially all the calcium contained in the residual liquor, the mixture is subjected to a separating operation, such as settling or filtering, to separate the precipitated calcium carbonate from the remaining liquid mixture. Such calcium carbonat is formed by reaction between soluble calcium compounds contained in the residual liquor and sodium carbonate contained in the added fixing agent or present in the residual liquor from a neutral or alkaline sodium sulfite cooking process in which the cooking liquor contains sodium carbonate. The amount of alkaline fixing agent added prior to this separating step may be all, or only part, of the total amount required to complete the subsequent fixing and pH adjusting treatment but, in order to cause substantially complete precipitation and separation of the calcium content of the residual liquor as calcium carbonate, the amount of fixing agent so added is preferably such as to introduce sufiicient alkaline sodium to provide a pH of at least 9 in the resulting mixture. If the residual liquor itself does not contain sufficient carbonate, the quantity of sodium carbonate in the fixing agent so added must also be such as to provide in the mixture sufiicient carbonate to combine with substantially all the calcium present to form calcium carbonate.

Following separation of the precipitated calcium carbonate, an additional quantity of the alkaline fixing agent may be introduced into the liquid mixture if desired. The mixture containing the remaining dissolved constituents of the residual liquor and the added fixing agent is then subjected to a fixing and pH adjusting treatment to cause oxidation-reduction reactions such as described above, between the sulfide compound of sodium introduced as a constituent of the fixing agent and the sulfur dioxide bearing constituents of the residual liquor, and to suitably adjust and stabilize the pH of the mixture as described above.- a 1 As illustrated in Fig. 2, this fixing and pH adjusting treatment may comprise heating the mixture under pressure in a suitable'digestor or other closed vessel provided with suitable means for heating the contents, as by direct introduction of steam at suitable pressure or by closed steam coils or in any other suitable manner. The temperature is'preferably maintained at about 300 F. or higher, corresponding to a developed steam pressure of about pounds per square inch (gauge) or more during this digesting treatment. Temperatures in the neighborhood of 320 F. to 325 F., corresponding to developed gauge pressures of about to pounds per square inch, have been found advantageous.

If desired, an additional amount of the alkaline fixing agent may be introduced into the mixture following the separation of theprecipitated calcium carbonate and prior to or during the heating under pressure. The alkaline fixing agent and the residual liquor are preferably both heated, either separately or after mixing, before being introduced into the vessel in which the digestion is carried out.

This heating under pressure may be continued for a sufficient period of time to convert most of the volatile sulfur compounds present to compounds that will not volatilize or decompose with evolution of H28 or other gaseous sulfur compound upon subsequent heating of the treated mixture to evaporate water. A period of about 30 minutes is generally sufficient.

When the residual liquor is mixed with alkaline fixing agent containing a sulfide compound of sodium and the mixture is heated under pressure as described above, the added sulfide compound reacts, either directly or indirectly, with sulfite' and ligno-sulfonic compounds in the liquor to form compounds that are non-corrosive and that will not volatilize or decompose with evolution of gaseous sulfur bearing products upon heating.

In some cases, as shown in Fig. 2, the fixing and pH adjusting treatment to prevent loss of volatile sulfur compounds and to adjust and stabilize the pH of the mixture may be effected advantageously, in whole or in part, by aeration of. the mixture of residual liquor and the alkaline fixing agent to cause alteration of the composition and stabilization or fixing of the sulfur. This aeration treatment may be carried out following or during the addition of such alkaline fixing agent, and comprises contacting the mixture with air in such manner as to provide an extended surface of contact of the mixture with the air. It may be carried out, for example, by passing the mixture through a tower in which it is distributed and brought into contact with a stream of air, or by blowing air through a quantity of the mixture in a suitable tank, preferably through a porous diffusing medium.

If the fixing and pH adjusting treatment is effected by aeration without previous heat treatment under pressure, an additional quantity of the alkaline fixing agent may be added to the mixture following separation of the precipitated calciumcarbonate and prior to the aeration of the mixture, or in one or more increments during the aeration, or partly prior to and partly during the aeration.

This aeration treatment may be carried out at or above atmospheric temperature, and is preferably conducted at a temperature above atmospheric, for example, at about 150 to F. It is preferably carried out at substantially atmospheric pressure, but may be conducted at pressures somewhat above or below atmospheric. The time required to effect the desired fixing and pH adjustment of the liquor in this manner is dependent upon various factors, such as the quantity of sulfide compound of sodium added, the temperature of treatment, and the rate and method of aeration employed. For example, when air is introduced at the bottom of a body of the mixture at a depth of about 2 feet below the liquid level, through a finev porous diffusion medium ata' rate of" about 1 /2 to 2 cubic feet per square foot of diffuser area, the treatment period generally varies between about 20 minutes and about 2 hours. Under these conditions the reactions proceed rapidly, as indicated by a marked rise in temperature of the mixture despite the cooling effect of evaporation of water.

As noted above, the alkaline fixing agent containing a sulphide compound of sodium and sodium carbonate may be introduced into the residual liquor prior to the fixing and pH adjusting treatment, or at one or more points during such treatment, or partly prior to and partiy in one or more increments during such treatment. It wili also be understood that where the fixing agent is introduced at successive points, the composition of the fixing agent introduced at the different points may vary; for example, the quantity of fixing agent introduced at one or more points may contain a sulphide compound of sodium with relatively little or no sodium carbonate, and the quantity introduced at one or more other points may contain sodium carbonate with relatively little or no sulphide compound. It will be further understood that the statements herein with respect to the amounts of sulphide compound and sodium carbonate in the fixing agent in proportion to the tetravalent sulfur in the residual liquor refer to the total amounts in all the fixing agent added prior to or during the fixing and pH adjusting treatment. It should be noted, however, that when the fixing and pH adjusting treatment comprises an aeration treatment such as described above, the mixture should be maintained at least slightly alkaline throughout such aeration treatment.

According to a preferred procedure, the fixing and pi-i adjusting treatment comprises both heating under pressure and aeration, the mixture of the residual liquor and added alkaline fixing agent being preferably first heated under pressure and then subjected to contact with air, as illustrated in Fig. 2. If desired, part of the alkaline fixing agent may be introduced into the solution prior to or during the pressure digestion and part of it may be introduced after the pressure digestion and prior to or during the aeration treatment.

For example, especially good results have been ob tained by adding to the residual liquor a solution containing NazS and NazCOs in such amounts as to provide about 1 part by weight of sulfur in the added solution to each part by Weight of tetravalent sulfur in the liquor, and to provide a total of about 2% parts by weight of alkaline sodium in the mixture (including the sodium in the added NasS and NazCOa and the alkaline sodium, if any, in the residual liquor) for each part by weight of tetravalent sulfur in the residual liquor; separating the precipitated calcium carbonate; heating the resulting liquid mixture under pressure at a temperature of about 320 to 325 F. for about 20 minutes, to cause rapid reaction between sodium sulfide and sulfur dioxide bearing compounds of the residual liquor, so as to release most of the sulfur dioxide from the organic sulfonic compounds of the residual liquor and effect partial conversion of the volatile sulfur compounds to non-volatile compounds by oxidation-reduction reactions, and also effect partial adjustment of the pH of the mixture; and then blowing the mixture with air for about 20 to 30 minutes at atmospheric pressure and at a temperature of about 180 to 200 15., to cause further oxidation-reduction reactions between sulfur dioxide bearing compounds and sulfide compounds and complete the removal of volatile sulfur compounds, and to complete the adjustment and stabilization of the pH of the mixture at a value between and 11.

The following specific examples are illustrative of the above described fixing and pH adjusting treatment following addition of the fixing agent:

Example I Residual liquor from sodium base acid sulfite cooking 10 process, having a pH of 3.2 and containing about 10% by weight of total solids including 0.77% tetravalent sulfur in sulfur dioxide bearing compounds, 0.55% nonalkaline sodium, and about 0.05% calcium.

A fixing agent comprising an aqueous solution containing 18% NazS and 12/270 NazCOs was added to the residual liquor in an amount equal to 10.5% by weight of the residual liquor. The precipitated calcium carbonate formed was separated by settling, to provide a soltuion containing less than 0.005% calcium.

This solution was heated under pressure at a temperature of 320 to 325 F. for approximately 20 minutes; it was then cooled to about 160 F. and blown at atmospheric pressure with air introduced through a finely porous diffusing medium at the rate of 2 cubic feet per minute per square foot of diffusing area, at a depth of 2 feet below the liquid level, for 20 minutes. During this treatment the solution was substantially converted to a foam, and the temperature increased to about 180 F.

The air blowing was then discontinued, and the foam allowed to collapse. The resulting treated mixture had a pH of about 10.8 and showed by test a loss of less than A of its sulfur content upon evaporation of water to 65% solids.

Example II Residual liquor from neutral sodium sulfite semichemical cooking process, having a pH of about 7 and containing about 15% by weight of total solids by weight including 1.18% tetravalent sulfur in sulfur dioxide bearing compounds, 0.85% non-alkaline sodium, and 1.28% alkaline sodium.

A fixing agent comprising an aqueous solution containing 11.3% NazS and 1% NazCOs was added to the residual liquor in an amount equal to 25% by weight of the residual liquor, and the mixture, at a temperature of about 150 F. and at atmospheric pressure, was blown with air in the same manner and at the same rate as in Example I above, for a period of 1 hour, during which the liquid was substantially converted to a foam and the temperature increased to about 180 F.

The air blowing was then discontinued, and the foam allowed to collapse. The resulting treated mixture had a pH of about 11.6 and showed by test a loss of less than A of its sulfur content upon evaporation of water to 65% solids.

Following the fixing and pH adjusting treatment as described above, the treated mixture may, if desired, be filtered to separate suspended solids, and is then heated to evaporate water therefrom and produce a concentrated mixture containing from to 70% solids, and preferably about to 70% solids. This evaporation may be carried out in a multiple effect evaporator of the type customarily employed for concentration of black liquor in a sulfate process pulp mill.

However, in order to concentrate to 60% solids or above, the last stage of the evaporation is preferably con ducted in a forced circulation evaporator. The treated mixture subjected to evaporation is less corrosive, or no more corrosive, than ordinary sulfate process black liquor, and no appreciable release of gaseous sulfur hearing products or other corrosive gases occurs during the evaporation. When the mixtures produced by treatment as described in the specific examples given above were concentrated by evaporation to solids, the loss of sulfur due to volatilization during the actual evaporation was less than A% of the contained sulfur.

The concentrated mixture is then burned in a suitable furnace of a type that provides combined burning and smelting, for example, a recovery furnace of the type presently used for burning concentrated black liquor in sulfate process pulp mills, to form a smelt consisting principally of NazS and NazCOs and to produce hot combustion gases containing part of the sulfur content of the concentrated liquor, principally in the form of S02. For example, the concentration and burning of treated mixtures produced in accordance with the above description has been found to produce a smelt having approximately 50% sodium sulfide, 5% sodium sulfate, and 45% sodium carbonate, all on the basis of sodium content, and combustion gases containing about 0.6 to 0.7% sulfur dioxide by volume. When the fixing agent intro duced into the residual liquor contained approximately 1 part of sulfur as sodium sulfide, and the total alkaline sodium in the mixture was approximately 2% parts, for each part of tetravalent sulfur in the residual liquor, the amount of smelt produced contained an amount of sodium sulfide approximately equal to that contained in the fixing agent, while the sulfur given off as sulfur dioxide in the combustion gases was approximately equal to the amount of tetravalent sulfur in the residual liquor.

The hot combustion gases are cooled as by passing them through a heat exchanger to heat the air going to the furnace, and are treated to recover suspended solids therefrom, preferably by means of an electrical precipitator. he gases are then scrubbed with sodium sulfite solution in a suitable scrubbing tower, to recover SOz therefrom and form a solution containing sodium acid sulfite, which may be used in the production of cooking liquor suitable for re-use in the cooking process.

The molten smelt discharged from the furnace is preferably treated to recover a sulfide compound of sodium and a carbonate compound of sodium therefrom. For example, as shown in Fig. 2, the smelt may be mixed with sufficient water to dissolve both the sodium sulfide and the sodium carbonate, and the resulting green liquor may then be filtered, preferably after the addition of lime or other filter aid. The clarified green liquor may then be subjected to evaporation, preferably in a de-salting evaporator, to evaporate part of the water and cause precipitation of sodium carbonate which is separated from the mother liquor containing sodium sulfide, as by filtration or centrifugal separation accompanied or followed by washing to provide a solid product containing sodium carbonate substantially free from sodium sulfide.

Part of the separated mother liquor containing sodium sulfide is preferably returned to the green liquor evaporator to control the ratio of sodium sulfide to sodium carbonate in the evaporator. By keeping the liquor in the evaporator hot and approaching saturation with respect to sodium sulfide, substantially all or a major part of the sodium carbonate can be precipitated and separated from a mother liquor containing substantially all the sodium sulfide with only a relatively small amount of sodium carbonate.

However, since the mother liquor containing sodium sulfide is preferably returned for use in the fixing agent, and since the fixing agent preferably also contains sodium carbonate, we prefer to so control the operation of the green liquor evaporator as to leave a suitable proportion of the sodium carbonate dissolved in the mother liquor along with the sodium sulfide.

All or part of the separated mother liquor containing sodium sulfide, in excess of the portion returned to the green liquor evaporator, is preferably returned and used in the fixing agent for the treatment of the residual liquor prior to concentration and burning, as described above. Part of this mother liquor may, if desired, be introduced into the concentrated liquor prior to burning,

The separated sodium carbonate, preferably after being dissolved in water, may be mixed with part or all of the solution containing sodium acid sulfite from the gas scrubbing operation to form sodium sulfite with evolution of carbon dioxide, thus regenerating sodium sulfite in solution for use in the scrubbing operation.

When the above described embodiment of the recovery process is applied to the treatment of residual liquor from a sodium base acid sulfite cooking process, using approximately 1 part of sulfur as sodium sulfide in the fixing agent for each part of tetravalent sulfur in the residual liquor, the fixing agent preferably also contains about /2.

part of sodium in the form of carbonate for each part of sodium in the formof sulfide. It is therefore advantageous in such cases to so control the operation of the green liquor evaporator as to leave approximately half of the sodium carbonate from the smelt dissolved along with the sodium sulfide in the mother liquor withdrawn for use as a fixing agent for treating the residual liquor. The solid sodium carbonate separated from the mother liquor is then approximately the amount required for use in effecting substantially complete recovery of sulfur dioxide from the furnace gases produced in the burning operation, to regenerate sodium acid sulfite for use in the cooking process. Thus, according to the preferred embodiment of the process illustrated in Fig. 2 as applied to residual liquor from an acid sulfite cooking process, this separated solid sodium carbonate recovered from the smelt is dissolved in Water and mixed with part of the sodium acid sulfite solution from the gas scrubbing operation to form sodium sulfite which is returned to the gas scrubbing operation to recover the sulfur dioxide from the furnace gases and form additional sodium acid sulfite which is diverted from the solution leaving the scrubbing operation, as indicated in dot-dash lines, and returned for use in the cooking liquor.

When the above described embodiment of the recovery process is applied to the treatment of residual liquor from a neutral or alkaline sodium base sulfite cooking process such as the neutral sulfite semi-chemical process, which residual liquor contains some alkaline sodium, the fixing agent preferably contains some sodium carbonate along with the sodium sulfide, but the proportion of sodium carbonate required in the fixing agent to establish a desired pH during the fixing treatment is less than in the treatment of acid sulfite waste liquor. When the fixing agent contains approximately 1 part by weight of sulfur as sodium sulfide for each part of tetravalent sulfur in the residual liquor, the proportion of sodium carbonate in the fixing agent is preferably such as to provide only a small fraction, such as about A part or less, of sodium in the form of carbonate for each part of sodium in the form of sulfide. It is therefore advantageous in such cases to so control the gree liquor evaporation as to leave only a relatively small proportion, for example about onetenth, of the sodium carbonate from the smelt dissolved along with the sodium sulfide in the mother liquor withdrawn for use as a fixing agent in the treatment of the residual liquor. The major portion of the sodium carbonate from the smelt is thus precipitated during the evaporation of the reen liquor and recovered by separation from the mother liquor as described above, and is used in the recovery of sulfur dioxide from the furnace gases produced in the burning operation, to regenerate sodium sulfite for use in the cooking process. In the process illustrated in Fig. 2, as applied to the treatment of neutral or alkaline sulfite residual liquor, this separately recovered portion of the sodium carbonate is dissolved in water and mixed with the sodium acid sulfite solution from the gas scrubbing operation to form so dium sulfite, a portion of which is returned to the gas scrubbing operation to recover the sulfur dioxide from the furnace gases and regenerate sodium acid suliit'e, while the remaining portion of the sodium sulfite is diverted from this part of the process, as indicated in dotted lines, and returned for use in the cooking liquor. If required, an additional amount of sodium carbonate may be introduced into this part of the process, for example, by mixing with the sodium acid sulfite solution along with the sodium carbonate recovered from the smelt, or by introduction into the sodium sulfite solution or into the ortion thereof returned to the gas scrubbing operation or diverted for re-use in the cooking liquor.

The above described recovery process may be applied advantageously to the treatment of combined residual liquor from different sodium base sulfite cooking processes. For example, it may be used for treating the combined residual liquor from a sodium base acid sulfite process and a neutral sulfite semi-chemical process, which contains sodium sulfite and may also contain either sodium acid sulfite or a non-sulfur bearing alkaline sodium compound, depending upon the particular compositions and relative amounts of residual liquor from the respective cooking processes. In such cases also, a portion of the sodium carbonate from the smelt is preferably returned along with the sodium sulfide for use in the fixing agent introduced into the combined residual liquor, and the remainder of the sodium carbonate is recovered separately and used in the recovery of sulfur dioxide from the furnace gases. When this sulfur dioxide recovery is carried out by scrubbing the gases with sodium sulfite solution to form sodium acid sulfite and using the sodium carbonate to react with acid sulfite so produced to form sodium sulfite, as shown in Fig. 2, a suitable portion of the sodium sulfite solution so formed is preferably diverted as indicated in dotted lines for use in the neutral sulfite semi-chemical cooking liquor, and a suitable portion of the sodium acid sulfite solution from the gas scrubbing operation is preferably diverted as indicated in dotdash lines for use in the acid sulfite cooking liquor.

An alternative method of separating sodium sulfide and sodium carbonate contained in the smelt produced in the above described process is illustrated in Fig. 3. In this method the smelt is subjected to a selective dissolving operation to dissolve the sodium sulfide while leaving at least a substantial proportion of the sodium carbonate undissolved. This may be done by introducing the molten smelt in very finely dispersed condition into a hot slurry containing sodium sulfide in aqueous solution and undissolved sodium carbonate, and by maintaining a high concentration of sodium sulfide in the solution. A portion of the hot slurry is withdrawn from the tank or other vessel in which this selective dissolving is carried out, and treated to separate undissolved sodium carbonate from the sodium sulfide solution, as by filtration or sedimentation accompanied or followed by washing to provide a sodium carbonate product substantially free from sodium sulfide.

A portion of the separated sodium sulfide solution is preferably returned to the slurry in the dissolving vessel, along with added water, to dissolve additional sodium sulfide and maintain the desired concentration of sodium sulfide. All or part of the sodium sulfide solution, in excess of that returned to the selective dissolving operation, is withdrawn and returned for use as a fixing agent in the treatment of the residual liquor from the cooking process, as described above. The amount of water introduced into the dissolving vessel is preferably so controlled as to dissolve a desired proportion of the sodium carbonate from the smelt, along with the sodium sulfide, so that the solution Withdrawn and returned for use in the fixing operation contains a suitable proportion of sodium carbonate as well as substantially all of the sodium sulfide from the smelt.

The separated sodium carbonate recovered. in solid form from the selective dissolving operation may be dissolved in water and the resulting solution is preferably subjected to filtration or other suitable treatment to remove insoluble solids derived from the smelt. The sodium carbonate solution may then be used in the recovery of sulfur dioxide from the furnace gases and the production of sodium acid sulfite or sodium sulfite, or both, for use in the cooking process, in the same manner as described above in connection with Fig. 2.

Another alternative method of treating the smelt from the furnace is illustrated in Fig. 4, in which the smelt may be mixed with water to dissolve both the sodium sulfide and the sodium carbonate, and the solution filtered to separate undissolved solids, in the same manner as described in connection with Fig. 2. In this case, however, sodium bicarbonate is introduced into the resulting clarified green liquor in a suitable reaction vessel, preferably 14 at or about atmospheric temperature. The mixture is preferably agitated and maintained saturated with respect to carbonate while keeping the sodium sulfide in solution. .The sodium bicarbonate reacts with all or part of the sodium sulfide in the solution to form sodium hydrosulfide and sodium carbonate.

The formation of additional sodium carbonate by this reaction causes precipitation of hydrated sodium carbonate or sodium sesquicarbonate or both. The sodium hydrosulfide remains in solution in the mother liquor, which is separated from the precipitated carbonate compounds as by centrifuging accompanied or followed by washing to provide a solid product containing sodium carbonate substantially free from a sulfide compound of sodium.

The separated mother liquor containing sodium hydrosulfide, with or without some unreacted sodium sulfide, and also containing some sodium carbonate, is preferably returned and used as a fixing agent in the treatment of residual liquor from the cooking process as described in connection with Fig. 2.

The separated sodium carbonate product may be mixed with a solution containing sodium acid sulfite, such as all or a portion of the solution produced in the gas scrubbing operation as in Fig. 2 in such proportion as to cause reaction of sodium carbonate with sodium acid sulfite to form sodium bicarbonate in solid form and sodium sulfite in solution. The resulting solution containing sodium sulfite may be separated from the solid sodium bicarbonate and used for scrubbing the furnace gases to recover S02 and form sodium acid sulfite, and the separated sodium bicarbonate may be returned and mixed with the clairfied green liquor for reaction with sodium sulfide as described above. A portion of the sodium acid sulfite solution from the gas scrubbing operation, or a portion of the sodium sulfite solution produced by reaction thereof with sodium carbonate, or a portion of each of these solutions, may be diverted and used in the production of sodium base sulfite cooking liquor, in the same manner as described above in connection with Fig. 2.

Another modified method of treating the smelt from the furnace is illustrated in Fig. 5. This method involves a selective dissolving operation which may be carried out in a manner similar to that described in connection with Fig. 3, except that the molten smelt is subjected to oxidizing treatment prior to or in conjunction with the dissolving step. For example, the molten smelt in finely dispersed condition may be brought into contact with air or other oxidizing agent prior to mixing with the hot slurry containing undissolved sodium carbonate, in the dissolving vessel, to oxidize sodium sulfide and form other sulfur compounds of sodium, such as thiosulfate, polysulfides or polythionates. These oxidized sulfur compounds are dissolved, while at least a substantial portion of the sodium carbonate remains undissolved, the amount of water introduced into the dissolving vessel being preferably so controlled as to dissolve a suitable proportion of the sodium carbonate along with the sulfur compounds. The resulting solution may also contain some sodium hydroxide, and may also contain some sodium sulfite or sulfate. A portion of the hot slurry is withdrawn from the dissolving vessel and centrifuged or otherwise treated to separate undissolved sodium carbonate from the solution. A portion of the separated solution may be returned to the slurry in the dissolving vessel to assist in maintaining the desired concentration of dissolved sulfur compounds.

In addition to oxidized sulfur compounds such as mentioned above, the separated solution may also contain some unreacted sodium sulfide. The principal sulfur compounds therein contain sulfur in a lower oxidation state than tetravalent, and part or all of this solution, other than that returned to the selective dissolving vessel, may therefore be returned andmixed with the residual liquor from the cooking process, in the fixing of pH 15 adjusting treatment prior to concentration andburning, in substantially the same manner as described in connection with Fig. 2.

The separated sodium carbonate may be dissolved in water and filtered to remove undissolved solids, and the resulting solution may be utilized in the recovery of sulfur dioxide from the furnace gases and the production of a sulfite compound of sodium for re-use in the cooking process; for example, this sodium carbonate solution may be reacted with sodium acid sulfite to form sodium sulfite solution which may be used in scrubbing the furnace gases to recover sulfur dioxide and form additional sodium acid sulfite, as described above in connection with Fig. 2.

We have described, by way of example, different methods of carrying out the treatment of the residual liquor to alter the chemical composition thereof prior to concentrating and prior to burning of the concentrated liquor, and different methods of treating the smelt from the furnace to recover useful compounds of sodium and sulfur therefrom. It is to be understood that these different methods, or portions thereof, may if desired be combined with each other in order to provide a process that is best adapted to the operations of a particular plant and the most economical and effective utilization of the recovered products.

We claim:

1. In the recovery of constituents of residual aqueous cooking liquor derived from the treatment of ligninbearing cellulose materials with sodium base sulfite wherein said residual cooking liquor contains inorganic tetravalent sulfur compounds including NaHSOs and contains also compounds of the class consisting of ligno-sulfonates containing NaHSOs; the process that includes oxidizing sulfurearing material in which the sulfur is chemically combined as sulfide to convert the sulfide sulfur to sulfur in an oxidation state lower than tetravalent, reacting the sulfur so produced with said tetravalent sulfur compounds in a quantity sufficient to deplete the liquor with respect thereto and thereby produce dissociation of said ligno-sulfonates and the release therefrom of NaHSOs, and continuing said reaction to the extent that the liquor is substantially completely so depleted and will not evolve appreciable amounts of sulfur-containing gases upon subsequent evaporation and concentration of the liquor, then after completion of said reaction of the sulfur with said tetravalent sulfur compounds heating the resulting liquor to evaporate water therefrom and form a concentrated mixture Without substantial evolution of sulfur-containing gases from the mixture during the concentration, and burning the concentrated mixture to release part of the sulfur content thereof in the combustion gases as sulfur dioxide and to produce a smelt containing sodium and most of the remaining sulfur content of the concentrated mixture.

2. in the recovery of constituents of residual aqueous cooking liquor derived from the treatment of ligninbearing cellulose materials with sodium base sulfite wherein said residual cooking liquor contains inorganic tetravalent sulfur compounds including NaI-ISO; and contains also compounds of the class consisting of ligno-sulfonates containing NaI-ISOs; the process that includes introducing to said liquor a fixing agent containing sulfur-bearing material in which the sulfur is chemically combined as sulfide, oxidizing said sulfide in the resulting mixture to convert the sulfide sulfur to sulfur in an oxidation state lower than tetravalent, reacting the sulfur so produced with said tetravalent sulfur compounds in a quantity sufficient to deplete the liquor with respect thereto and thereby produce dissociation of said lignosulfonates and the release therefrom of NaHSOs, and continuing said reaction to the extent that the liquor is substantially completely so depleted and will not evolve appreciable amounts of sulfur-containing gases upon subsequent evaporation and concentration of the liquor, then after completion of said reaction of the sulfur with said tetravalent sulfur compounds heating the resulting liquor to evaporate water therefrom and form a concentrated mixture without substantial evolution of sulfur-containing gases from the mixture during the concentration, and burning the concentrated mixture to release part of the sulfur content thereof in the combustion gases as sulfur dioxide and to produce a smelt containing sodium and most of the remaining sulfur content of the concentrated mixture.

3. The process as defined by claim, in which said reaction of the sulfur with said tetravalent compounds is carried to the extent of effecting adjusting and stabilization of the pH of the mixture at a value greater than 9.

4. The process as defined by claim 1, in which a mixture of said residual liquor and said sulfur-bearing material is heated under pressure at a temperature of at least about 300 F. to produce said sulfur in an oxidation state lower than tetravalent by oxidation reduction reaction of the sulfur-bearing material and part of said tetravalent compounds and to react the sulfur in the oxidation state lower than tetravalent with the remainder of said tetravalent sulfur compounds.

5. The process as defined by claim 1, in which a mixture of said residual liquor and said sulfur-bearing material is intimately contacted with air to oxidize said sulfur-bearng material to sulfur in an oxidation state lower than tetravalent.

6. The process as defined by claim 1, in which said sulfur-bearing material contains between /2 part and 1 /2 parts by weight of sulfide for each part by weight of'tetravalent sulfur in the residual liquor.

7. The process as defined by claim 1, in which said smelt contains sodium sulfide and sodium carbonate, and including the further steps of separating from said smelt most of the sodium sulfide content thereof in the form of a sodium compound containing sulfur in a state of oxidation lower than tetravalent, separately recovering at least part of the sodium carbonate content thereof in the form of a carbonate compound of sodium, and oxidizing the sulfide sulfur in the sulfur-containing sodium compound so recovered from the smelt to an oxidation state lower than tetravalent for reaction with the tetravalent sulfur compounds in a further quantity of residual liquor in said process.

8. The process as defined by claim 1, in which the sulfite in said sulfur-bearing material comprises sodium sulfide and said smelt contains sodium sulfide and sodium carbonate, the process including the further steps of separating from the smelt most of the sodium sulfide therefrom and separately at least part of the sodium carbonate and converting the sulfur in the sodium sulfide so recovered to an oxidation state lower than tetravalent for reaction with said tetravalent sulfur compounds in a further quantity of said residual liquor in said process.

9. The process as defined by claim 2, in which said oxidation of the sulfide and reaction of the resulting sulfur with said tetravalent sulfur compounds are partially affected by heating the mixture of said liquor and fixing agent under pressure at a temperature of at least about 300 F. and thereafter substantially completed by intimately contacting the mixture with air.

10. The process as defined by claim 2, in which said fixing agent contains also alkaline sodium.

11. The process as defined by claim 2, in which said fixing agent contains between /2 part and 1 /2 parts by weight of sulfur combined as sulfide for each part by weight of tetravalent sulfur in the residual liquor, and also contains alkaline sodium in such amount that the total alkaline sodium in the fixing agent and in the residual liquor equals between about 1 /2 parts and about 3 parts by weight for each part of tetravalent sulfur in the residual liquor.

12. The process as defined by claim 2, in which said 17 fixing agent comprises a sulfide compound of sodium and sodium carbonate.

13. The process as defined by claim 2, in which the sulfide compound of the fixing agent comprises sodium sulfide, the fixing agent contains sodium carbonate, and the smelt contains sodium sulfide and sodium carbonate, the process including the further steps of separating from said smelt most of the sodium sulfide therefrom together with part of the sodium carbonate and separately the rest of the sodium carbonate, and introducing the sodium sulfide and sodium carbonate so recovered together from the smelt into a further quantity of said residual liquor in said process.

14. The process as defined by claim 7, in which the carbonate of sodium recovered separately from the smelt is reacted with sulfur dioxide in said combustion gases to produce a sulfite compound of sodium suitable for use as sodium base sulfite cooking liquor.

15. The process as defined by claim 7, comprising the further step of reacting the carbonate compound of sodium recovered from the smelt with sodium acid sulfite 18 in aqueous solution to form a solution containing sodium sulfite, and bringing the combustion gases from the burning operation into contact with said solution to recover sulfur dioxide from said gases and form sodium acid sulfite.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Paper Trade Journal, pages 27-31, July 4, 1952. 

1. IN THE RECOVERY OF CONSTITUENTS OF RESIDUAL AQUEOUS COOKING LIQUOR DERIVED FROM THE TREATMENT OF LIGNINBEARING CELLULOSE MATERIALS WITH SODIUM BASE SULFITE WHEREIN SAID RESIDUAL COOKING LIQUOR CONTAINS INORGANIC TETRAVALENT SULFUR COMPOUNDS INCLUDING NAHSO3 AND CONTAINS ALSO COMPOUNDS OF THE CLASS CONSISTING OF LIGNO-SULFONATES CONTAINING NAHSO3; THE PROCESS THAT INCLUDES OXIDIZING SULFUR-BEARING MATERIAL IN WHICH THE SULFUR IS CHEMICALLY COMBINED AS SULFIDE TO CONVERT THE SULFIDE SULFUR TO SULFUR IN AN OXIDATION STATE LOWER THAN TETRAVALENT, REACTING THE SULFUR SO PRODUCED WITH SAID TETRAVALENT SULFUR COMPOUNDS IN A QUANTITY SUFFICIENT TO DEPLETE THE LIQUOR WITH RESPECT THERETO AND THEREBY PRODUCE DISSOCIATION OF SAID LIGNO-SULFONATES AND THE RELEASE THEREFROM OF NAHSO3, AND CONTINUING SAID REACTION TO THE EXTENT THAT THE LIQUOR IS SUBSTANTIALLY COMPLETELY SO DEPLETED AND WILL NOT EVOLVE APPRECIABLE AMOUNTS OF SULFUR-CONTAINING GASES UPON SUBSEQUENT EVAPORATION AND CONCENTRATION OF THE LIQUOR, THEN AFTER COMPLETION OF SAID REACTION OF THE SULFUR WITH SAID TETRAVALENT SULFUR COMPOUNDS HEATING THE RESULTING LIQUOR TO EVAPORATE WATER THEREFROM AND FORM A CONCENTRATED MIXTURE WITHOUT SUBSTANTIAL EVOLUTION OF SULFUR-CONTAINING GASES FROM THE MIXTURE DURING THE CONCENTRATION, AND BURNING THE CONCENTRATED MIXTURE TO RELEASE PART OF THE SULFUR CONTENT THEREOF IN THE COMBUSTION GASES AS SULFUR DIOXIDE AND TO PRODUCE A SMELT CONTAINING SODIUM AND MOST OF THE REMAINING SULFUR CONTENT OF THE CONCENTRATION MIXTURE. 